A fundamental property of animal cells is their ability to maintain a constant volume throughout life. In neurons, the possibility of changes in ionic composition and hence in cell volume is present at all times. This is a consequence of their small volume/surface ratio in conjunction with their synaptic and all-or-none electrical activity. Under physiologic conditions neurons are able to maintain and restore the ionic gradients which determine their resting membrane potential and osmotic balance. In spite of their importance, little is known about the mechanisms underlying cell volume regulation (CVR) and cell volume maintenance (CVM) in neurons. Furthermore, nerve cell swelling (cytotoxic edema), a dreaded complication of ischemia or trauma, has been postulated to result from loss of control of cell volume. Clearly further advances in pathophysiology of brain edema will require elucidation of the membrane mechanisms underlying CVR and CVM as well as the factors eliciting nerve cell swelling. This is a proposal to study the basic membrane mechanisms with which neurons are .equipped to regulate and maintain their volume in isosmotic and anisosmotic media. We aim to ascertain how nerve cells respond to hyposmotic and hyperosmotic challenges , whether they are endowed with mechanisms for regulatory volume adjustments, and what is the nature of these mechanisms. In isosmotic media we want to know: a) the role played by the Na+/K4+ pump in CVM, and the mechanism involved in volume regulation when the pump is inhibited; b) the role of anion transport in CVM; and c) whether cell volume can be altered by firing activity and how nerve cells restore their volume under these conditions. These studies will be conducted on identified land snail neurons and frog dorsal root ganglion cells. These cell types are chosen because: i) they are relatively large; ii) their cell bodies are nearly spherical; iii) many of their membrane carriers and channels have been characterized; iv) their intracellular ionic activities are known; v) they can be easily isolated and maintained in vitro for optical measurements; vi) they are examples of an invertebrate and a vertebrate neuron. Cell volume changes and membrane voltage will be measured with microelectrodes, using intracellular TMA+ as a volume marker. Intracellular ion -activities will be measured with ion-selective microelectrodes.